Aging is associated with decreased efficacy of vaccination, and increased morbidity and mortality following infection by agents such as pneumococcus, influenza and West Nile virus. Multiple acquired defects may contribute to the reduced ability of elderly individuals to produce protective antibodies against newly encountered pathogens. In aged mice, follicular (FO) B cells responsible for generation of protective antibodies to primary infection are reduced in numbers, and this is correlated with reduction in the FO B cell generative capacity of hematopoietic stem cells (HSCs). Thus, decreased B lymphopoiesis may be causal in the aging- associated decline in humoral immunity to newly encountered pathogens. A similar defect appears to exist in humans where transplanted bone marrow (BM) stem cells from older adult human donors often fail to give rise to B cells. The molecular basis of this B lymphopause is unknown. It is our long-term goal to understand this defect and develop approaches to correct it. Although aged mice contain significantly more HSCs than their young counterparts, these cells exhibit dysregulation of gene transcription and selective impairment in the ability to give rise to B cells. Hematopoietic cell development from HSCs proceeds through an ordered sequence of expression/function of transcription factors, which control lineage specification, commitment and progression. This sequence of events is controlled, in part, by chromatin accessibility of the genes involved in these processes. Chromatin accessibility is regulated by histone modification (e.g. acetylation, methylation, phosphorylation, etc.) and methylation of DNA CpG dinucleotides. Little is known regarding the patterns of histone modification and DNA methylation that impact early steps in hematopoiesis, and how these patterns might be altered during aging. Here we propose to address the hypotheses that dysregulation of transcription and defective B cell generative capacity of aged HSCs is the consequence of epigenetic changes and attendant genomic destabilization. Our approach will involve the use of ex vivo HSCs as well as conditionally transformed HSC lines derived from immunologically aged or young mice. The latter retain the functional phenotype of the originating adult stem cells, and provide an unlimited supply of material for biochemical studies. Further, these cells can be manipulated genetically to establish the role of specific alterations in HSC defects. B cell development from these HSCs will be studied both in vivo and in the OP9 culture system in which aging-associated B lymphopoietic defects are replicated. PUBLIC HEALTH RELEVANCE: A proportion of aged individuals suffer poor quality of life due to acquired immune defects that reduce their ability to fight infection. Such people require increased medical care and hospitalization, impacting public health primarily in economic terms. Our studies will attempt to define the basis of this immune decline and strategies to correct it.